1. Field Of The Invention
The field of the invention is that of optical devices for measuring the orientation of an object in space without contact. Various fields of possible applications exist, but the main application is the detection of the orientation and the position of an aircraft pilot's helmet, thus enabling an image to be projected in his visor in exact superposition onto the external landscape, or to lock different systems of the apparatus onto his gaze. The desired precision in such systems is in the order of the milliradian.
2. Description of the Prior Art
Different optical techniques exist for measuring the orientation on the helmet. Generally, distinctive elements which are located by a system of cameras are installed on the helmet. The position of the images of these distinctive elements allows the orientation of the helmet to be determined through calculation.
These elements may be passive or active. The passive elements are illuminated by an external source. Retroreflective cube corners or retroreflectors can be used for this purpose. It suffices to dispose the optical transmitting and receiving components on the same axis.
These retroreflector systems are insensitive to solar illumination, but are combined with one of the following types of fixed devices:                a camera which supplies an image, the quality of which degrades during movements of the helmet;        a combined lens for illumination and image capture which provides a wide depth of field but of which the size is significant.        
Moreover, in these two types of system, the limited angular field of the fixed device in the cabin reduces the range of possible positions of the helmet and the position measurement requires the presence of at least three reflectors on the helmet, thus adversely affecting the compactness of the system.